As disclosed at page 111 of the "'85 edition of Switch Latest Technology" published by Sogo Gijutsu Syuppansha on Sep. 28, 1984, a membrane switch comprises a spacer between two base films to define a space between the two base films and contact points (electrodes) arranged on the opposed surfaces of the two base films which face each other with the space in between. By depressing one of the base films, two contact points are brought into contact with each other to conduct electricity and by releasing the depressed base film, they are disconnected from each other, thus effecting a switching function.
In recent years, such a membrane switch has been frequently used as a key board switch for calculators and personal computers, a panel switch for the remote controller of TVs and VTRs, and the like.
Since this membrane switch effects a switching function by alternating being depressed and released, flexibility, that is, deformation resistance is required for the base film of the membrane switch. Heretofore, a polyethylene terephthalate (may be abbreviated as "PET" hereinafter) film has been generally and frequently used as the base film of this membrane switch due to its deformation resistance, adhesion to electrodes, adhesion to printing and the like.
However, owing to the recent use of touch panels for car audio systems and car air conditioning systems or remote control switches in a vehicle due to the popularization of car navigation systems, deformation resistance at high temperatures is required for the base film of the membrane switch.
When a PET film is used as the base film of the membrane switch as before, temperature inside a vehicle (about 80.degree. C.) may exceed the glass transition temperature of PET in summer and the membrane switch may malfunction due to the great deformation of a PET film.
To prevent this, JP-B 4-75610 proposes that a polyethylene naphthalene dicarboxylate (may be abbreviated as "PEN" hereinafter) film which has a higher glass transition temperature than a PET film be used as the base film in place of the PET film.
That is, JP-B 4-75610 discloses a membrane switch having contact points on the opposed surfaces of two base films, at least one of which is a biaxially oriented polyethylene naphthalene dicarboxylate film having an F-5 value (5% elongation stress) of 11 kg/mm.sup.2 or more, a density of 1.375 g/cm.sup.3 or less and a thermal shrinkage factor of 1.0% or less when heated at 120.degree. C. for 30 minutes.
JP-B 6-4276 discloses a polyester film for a membrane switch which is made from polyethylene naphthalate having a haze increase rate defined by the following expression of 20% or less when heated at 150.degree. C. for 2 hours: EQU haze increase rate=(H.sub.2 -H.sub.1)/H.sub.1 .times.100 (%)
wherein H.sub.1 is a haze value before the heat treatment and H.sub.2 is a haze value after the heat treatment, and thermal shrinkage factors in both longitudinal and transverse directions of 0.5% or less. PA1 (A) which is made from a polyester containing ethylene-2,6-naphthalene dicarboxylate recurring units in an amount of at least 80 mol %; and PA1 (B) which has an endothermic peak with an endothermic energy of at least 0.4 mJ/mg at a temperature of 110 to 160.degree. C. PA1 (1) the step of heating the film at a temperature of 150 to 180.degree. C. for 1 to 5 hours under no strain; and PA1 (2) the step of heating the film in an unrolled or rolled state at a temperature of 80 to 122.degree. C. for 5 to 200 hours.
However, standards for deformation resistance have been reviewed due to the influence of the recent enforcement of the PL Act and it has been pointed out that even a PEN film is unsatisfactory in terms of deformation resistance. When a membrane switch is used as a pressure sensor, especially when it is embedded in a seat of an automobile equipped with an air bag as a pressure sensor of a weight detection system for detecting pressure to alter the inflation speed of the air bag according to an adult or a child, a conventional PEN film has large residual deformation and cannot be used.